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The End Permian Mass Extinction

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IntroductionThink of a world which existed 290 million years ago. As you look out over the terane in front of you, you think that you are on an alien planet. You see volcanoes spewing ash and lava. Beside them is the ocean which is swarming with many different species of echinoderms, bryozoans and brachiopods. As you look down onto the sea floor you are amazed at the countless number of starfish and urchins. Some animals leave you can’t even describe and you have no idea even what phylum they belong to.

This is a world at its height in diversity of oceanic species. Millions of wonderous species existed at this time in the ocean and most of them will never appear again in earth’s history. In the geologic time scale, a million years means nothing but this time things are different. In the blink of an eye things now look vastly different. The world once again looks alien but it looks worse than before.

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The sky is dark. Oceans are no longer teaming with life. The stench of rotting flesh and plants hangs in the air. The ground trembles under your feet. You feel an intense heat burning you face. You look up and see one of the greatest show of force mother nature has ever shown. Whole mountains are being thrown in the air. Lava and debris are everywhere. You ask yourself, what has happened? Will life ever exist on earth again?The above paragraph is a primative example of what the end of the Permian period could have looked like. Marine life was devastated, with a 57% reduction in the number of families (Sepkoski, 1986) and an estimated 96% extinction at the species level (Raup, 1979). Oceanic life suffered the most but terrestrial life forms were also greatly affected. There was a 77% reduction in the number of tetrapod families (Maxwell and Benton, 1987). All major groups of oceanic organisms were affected with the crinozoans (98%), anthozoans (96%), brachiopods (80%) and bryozoans (79%) suffering the greatest extinction (McKinney, 1987). The end of the Permian and beginning of the Triassic periods marked the single greatest extinction event the world has ever faced.Timing of the ExtinctionThere are many questions regarding the timing of the extinction at the end of the Permian. One of the main questions was the even a catastrophy or gradual. There is evidence for both senarios. Some of the evidence supports an ectraterrestrial even such as a metior. Other evidence supports the theory of the ocean and terrestrial environments slowly changing.Geochemical evidenceThe research done by Xu Dao-Yi and Yan Zheng (1993)gives evidence for an extraterristrial event. They made a table which showed the distribution of carbon 13, iridium, and microspherules across the P/T (Permian and Triassic) border. The section was over a thickness of 35 cm. They found a sudden depletion in C-13 falling from a value near zero to a minimum of less than -6% in some samples. Similar patterns of C-13 have been observed in more than five P/T sections in China. Some other scientists like Baud et al (1989) argue that what could have caused this anomaly is the result of a depositional hiatus or erosional disconformity. Xu and Yan argue that there is no evidence for a significant hiatus and that Baud et al. Even made a mistake in the timing of their rock layers. “If the PTB Permian Triassic boundary is considered a catastropic event, a short-time hiatus should be expected and is in fact a reasonable consequence of a catastrophic event” (by Xu Dao-Yi and Yan Zheng, 1993). But what is the significance of C-13 being associated with catastrophic events? Hsu et al. (1982) said that they suggested that carbon isotope anomalies are related to microplankton productivity. We will touch again on this later in the paper. Therefore, the sudden C-13 change may indicate the exact stratigraphic position of the mass killing event at the PTB. Analysis of iridium (Xu Dao-Yi and Yan Zheng, 1993)in the layer reveild some interesting results. High Ir values only occurs in the uppermost part of the layers. This means that the layer is close to the PTB. The concentration of Ir was at least an order of magnitude higher than the background values and this is characteristic of most Upper Permian and Lower Triassic boundaries. The scientists go on to say that “the existance of a rich Ir anomaly on a global scale within the K/T boundary layers of both marine and continental facies has been interpreted as highly impressive evidence for an impact origin. Another discovery that may serve as a marker of an event is microspherules. A variety of microsherules have been discovered in the PTB layers of the Meishan section (Xu et al., 1989). The origin of the microspherules could be multiple. They are small circular indentations in the rocks and the most abondent elements are Si or Si-Al. Mircospherules are similar to cosmic dust. Since a large amount of microspherules occurs in a thin layer of PTB layer it can serve as another event marker.Maxwell (1989) who got his information from Clark et al. (1986) said thatThe elemental in boundary clays across China suggest that there is a remote possibility that the predominantly illite boundary clay is a remote possibility that the predomonantly illite boundary clay resulted from the alteration of ejecta dust from a comet impact, but the most likely source was ash from a massive volcanic eruption.The trace elements suggested that the dust was highly acidic and the ratios of TiO2 and AL2O3 are low enough to support the volcanic dust scenario (Clark et al. 1986).There is some research which gives evidence of a gradual extinction event. Magaritz et al. (1988) reported that carbon-isotope ratios are known to shift or change at some boundaries associated with a mass extinction event. A shift can occur due to a decrease in plant production following a meteor impact or from a large decrease in sea level that reduces shelf area, exposing the shelf and its accumulated organic carbon to erosion. There are sections examined in the Alps of Italy and Austria that actually show a gradual change in the C-13 content of marine organisms across the PTB. These sections show no dramatic shifts that can be associated with a mass extinction. Thus as you can see, the findings of Clark et al. (1985) and Magaritz et al. (1988) shows geochemical evidence that the mass extinction was a gradual event and not a catastrophic extinction event.Faunal evidenceFaunal evidence is much harder to come by and explain that geochemical evidence due to major gaps in the PTB boundary layers. Also marine faunal evidence is much more linear than terrestrial. Yoram Eshet et al. (1995) said that fungal evidence can be used to mark the PTB layer. It can also be used for evidence to show how the extinction event occurred. There is a sharp fungal spike in the PTB layer which is made up of Lueckisporites virkkiae, Endosporites papillatus, and Klausipollenites schaubergeri spores. Yoram Eshet et al. (1995) defined four stages across the Permian-Triassic boundary. Stage one, consisted of low abundances of spores which became increasingly abundent. At the top, the disapearance of more than 95% of the Late Permian pollen and spore taxa became apparent. Stage two contained and abunance of fungal remains and here it is defined as the “fungal spike”. Also there is quite a bit of organic detrius, composed of carbonized plant debris. Stage three and four will be described later in this paper. Since this fungal evidence can be seen throughout the world it makes it highly unlikely that the increase is everywhere an artifact resulting from sedimentary processes or local conditions. Also it should be noted that the fungal spike is very thin which suggests that remains could have been missed at many PTB layers. The reason there is a large fungal spike should be oviuos. Fungi are known to adapt and respond quickly to environmental stress and disturbance (Harris and Birch, 1992). During a high stress period, like an extinction event, decimation of autotrophic life occures which creates a large pool of decaying organic matter. This is evident by the abundent plant debris seen in the fungal spike.Marine evidence for the PTB extinction event has the greatest impact. According to Douglas H. Erwin (1993), the world’s leading expert on the Permian crisis, marine organisms such as bivalves and gastropods suffered the greatest so that most are unfamiliar even to students of invertebrate zoology. But findings by Erik Flugel and Joachim Reinhardt (1990) contains contradictory evidence that marine life suffered in the end Permian and early Triassic. It is commonly assumed that reefs are affected more severly at major extinction events than other biotopes. Another assumption is that there is a decrease in diversity of shallow-marine organisms during the Late Permian. In analylizing the Permian-Triassic reefs using very sophisticated equipment the scientists found that there was no reduction in diversity of reef organisms during the last part of the Permian. That there was evidence of high and even increasing diversity of the uppermost Permian reef comunities. The argument of Erik Flugel and Joachim Reinhardt (1990) was again countered by a number of scientists. Sweet (1992) showed that strata previously assigned to the topmost Permian stage was mistaken and that the strata should have been moved lower. If Sweet’s scheme is accepted, then the mass extinction becomes an intra-Triassic event. The differences in data could be due to inadequate sampling as proposed by Sepkoski (1986). The evidence for this statement is found in that there is virtually no complete late Permian sections and complete sections across the PTB layers. As you can see, nothing is fool-proof in the study of the Permian-Triassic extinction event. Since there is conflicting evidence of when, what, and how the extinction event occurred, there will be will be many different theories and hypothesis on the causes of the end Permian extinction. This paper will explore a few of the possibilities.One of most agreed with reasons of the cause of the extinction was made up by Newell (1963).Causes of the end Permian extinctionDiversity-DependentThere are many theoretical causes of the Permian mass extinction. The causes are divided up into two main groups: diversity-dependent and diversity independent. Diversity dependent hypotheses have just recently been formed and thus they are not very popular but they do make quite a bit of sense when looked at clearly. Diversity-dependent factors limit population growth as population size get larger. It involves a depleation of environmental factors such as oxygen, nitrogen, and carbon dioxide. Bramlette (1965) and Tappan (1968) evolved on a scenario of nutrient reduction. In the model, landscapes where flat and thus streams were not capable of transferring nutrients to the oceans. Also a reduction of upwelling activity helped the effect. They also proposed that oxygen levels may have declined as a result of a loss of primary productivity. Tappan went on to say that heavy extinction of suspension feeders at the end of the Devonian, Permian, and Cretaceous implicated changes in primary productivity as the main cause of the extinction through accumulation of organic material in the ocean and thus starving the ocean and land of nutrients. Once again let it be noted that the oceans would starve if there was no upwelling. Through this mechanism the end Permian is very gradual and it would selectively remove different species at different times. Many scientists critisize this mechanism because it would cause the oceans to be virtually sterile. Wingnall (1993) critisized this hypothesis by saying “It appears unlikely that the oxygen-deficiency was induced by high productivity for, as we have shown, organic-rich facies are only patchily developed in the Griesbachian early Triassic.”When thought through carefully, nutrient accumulation or sequestration would have reached a peak during the development of the extensive Carboniferous coal swamps and not during the Permian period. One very interesting hypotheses is based on biogeography. Erwin (1993) said that, Since most species occur only within a single marine province, one of the major controls on global diversity should be the number of marine provinces. Similar communities in different areas of a single province thend to have roughly similar community composition (at least for the more abundant species). Thus the species within a nearshore sandy-bottom community will tend to recur throughout a provonce but will differ between provinces.Since continents usually define marine boundaries then when continents are dispersed there will be more marine provinces and thus more diversity. Erwin goes on to say that the formation of Pangea (the great super-continent) in the late Permian times forced a reduction in sea-floor spreading.Since the depths of the ocean basins are a function of the age of oceanic crust, a reduction in the rate of sea-floor spreading will allow the mean age of the oceanic crust to increase, increasing the size of the ocean basins. The volume of the mid-ocean ridge spreading centers will also decline. The net effect should be a regression.Richard Leakey (1995) adds an interesting parrallel.Imagine four one-inch squares, each of which has a total edge length of four inches, giving a grand total of sixteen inches. Now bring them together as a single square of side two inches. The total edge length is now a mere eight inches, just half of the previous figure. The same thing happens with individual coninents and available shallow-water habitats. The formation of Pangea therefore must have devastated species in these habitats by this mechanism alone……Regression causes in increase in the continent’s surface area and it also alters climate patterns. There will be an increase in seasonality in nearshore waters along with an increase in nutrients and competition as provinces merge together. Therefor global diversity should be at its lowest when the supecontinent exists. The more continental climates and higher seasonality will increase the instability of nutrients, primary productivity, and other trophic resources. Here species that are affected seasonally will be affected greatest while species with a broad trophic and environmental tolerances will be favored. Since the study of instability if very complex we should treat these kinds of hypotheses carefully. In concluding, the above factors may well have played a role with other factors in causing the greatest extinction on the earth (Erwin, 1993).Diversity-IndependentNow we move away from diversity-dependent factors to diversity-independent hypotheses which are more common and accepted. This involves models that affect all individuals of a species equally and is independent of the number of species present. As mentioned before, most extinction fall into this category.ExtraterrestrialExtraterrestrial phenomena is one of the favorite explanations for the Permian extinction. There is quite a bit of evidence to support it. In Science News (1993), Monastersky reported on the findings of a Canadian team working with well-preserved shales and cherts from northeastern British Columbia. These rocks formed when the region lay at the bottom of an inland basin. The researchers got information about an ancient ocean during the Permian time by isolating from the rock small amounts of kerogen. Kerogen is the decomposed residue of Permian plankton. At the PTB the kerogen records drop sharply in the ratio between heavy C-13 atoms and light C-12 atoms. Monastersky goes on to sayTo interpret the shift in Carbon isotopes, the rsearchers exploited the fact that plants tend to avoid Carbon 13 as they grow during photosynthesis. Because of the vast number of phytoplankton competing for carbon-12 during normal times, however, the plants typically incorporate some carbon-13. But a sudden die-off of most phytoplankton would give survivors greater access to carbon-12. When they fall to the ocean floor and get incorporated into sedimentary rocks, they reduce the ratio of carbon-13 to carbon-12 within a rock….Geochemists who have studied inorganic carbon which came from shells of ancient plankton have also detected abrubt drops in carbon isotopic ratio at the end of the Permian. Due to the many factors that can alter this ratio they have not been able to isolate what caused the change. Fewer processes affect the carbon isotopic ratio in kerogen. This greatly strengthens the case that the surface ocean suffered from a biological crisis. “It is consistent with some sort of catastrophic event like an asteroid….”(Monastersky, 1993). Another paper witten by Richard Monasterky (1997) gives more evidence. It seems that a scientists named Gregory J. Retallack went searching in the Southern Hemisphere and reported that he found “shocked” quartz at two sites in the Antartica and one site in Australia. This type of quartz is riddled with intersecting sets of fractures and is born only during impacts. Iridium also adds to the evidence. Scientists now know that an impact caused the extinction at the Cretaceous-Tertiary boundary and there is also an increase in Iridium at this boundary. Thus they can deduce that if they find and increase of iridium at the PTB above background levels then they will have evidence of an impact. The discovery of a significant increase in iridium and microspherules at the PTB boundary by Xu Dao-Yi et al. (1993, 1985, 1989) gives imperical evidence of an impact.There are many problems with the impact theory. First there is evidence that shows that the Permian extinction started gradually and had a more rapid pulse at the end (Monastersky, 1993). Also some scientists have argued that the quartz cristals found by Retallack were not shocked because Retallack only studied them under a light microscope, where it is difficult to distinguish shock features from more prosaic deformations caused by normal tectonic stress in the Earth’s crust. Monastersky (1997) said that “an impact capable of triggering unparalleled losses should have strewn telltale clues around the world.” Western geologists have attempted to verify the Chinese reports of iridium and they are fruitless. Anomalies can cause a build up of iridium in one place which would lead to what the Chinese have discovered (Erwin, 1993). Cosmic RadiationGlobal CoolingGlobal WarmingSalinityThe hypothesis that salinity decrease caused the mass extinction of oceanic life was first formed by Beurlen in 1956 (Maxwell, 1989). Evidence for this phenomena was based mainly on stenohaline groups such as the bryozoans, ostracodes and corals which were greatly reduced at the PTB. The least affected groups were gastropods and fresh water fishes. Organisms with some tolerance of salinity variations survived and proliferated in the early Triassic. Therefore it was found that a selective extinction of marine families occurred in the BTB. Beurlen proposed that salinity was progressively reduced during the second half of the Permian and also that salinity reached critically low values at the PTB, before persisting into the early Triassic. Early marine faunas are sparse and many groups that were diverse before and after the PTB are not present at the PTB. Beurlen said that this was due to a few places in the world where normal salinities were maintained. A return to normal salinities world-wide would allow the surviving species to repopulate the seas and as a result, crop up again in the fossil record after their temporary absence. This leaves us with one main question, what would cause such a large reduction in ocean salinity? Maxwell (1989) gives some answers based on the work of many scientists. In the 1950’s and 60’s it was thought that the drop in salinity was due to large-scale evaporite sedimentation accompanied by the formation of large quantities of dense brine which was stored deep down on the sea floor. Salinity could have been reduced to a value around 30 parts per thousand (which is safe to drink). If this occurred than the result would be huge volumes anhydrite, gypsum, salt, and halite deposited on the sea floor. Beurlen (1956) estimated that 5*10^14 tonnes would need to be deposited. Other scientists strongly criticized Beurlen stating that this would only be 15% of the amount of evaporites that would need to be stored. A figure of 200,000 cubic kilometers was postulated but some scientists say that this is only 10% of the real amount. Therefor it would seem that Permian evaporite deposits can not explain the lowering of salinity levels. The best reason that I could find to explain salinity decreases was put forward by Fisher (1963) called the brine-reflux hypothesis. The evaporation of sea water and the deposition of salts produced dense brines which sank deep onto the floor of the ocean. This leaves the top circulating water free if salt. In looking at this proposal carefully, I think Fisher would come into opposition with scientists say that the extinction was due to a temperature decrease. A temperature decrease would cause less evaporation and should cause the oceans to be saltier due to fresh water being accumulated in glaciers. Erwin (1993) said that a scientists named Bowen in 1968 actually argued that Permian climates triggered an increase in Permian salinity of approximately 20% above today’s levels. His study was based on the volume if massive Louann salt deposits from the Gulf Coast and other Paleozoic evaporites. So as you can see there is a great deal of uncertainty if even salinity had anything to do with the PTB extinction. Erwin goes on so slam all of the hypotheses.These salinity hypotheses are instructive examples of how often “explanations” are nothing of the kind. Stenohaline taxa are also largely stebotopic, and often independent evidence must be advanced that salinity changes were the selective factor. Contrary to several of these papers, nautiloids did not particularly suffer during the extinction, blastoids and crinoids disappeared long before the ammonoiads or the brachiopods, and “strophomenid” brachiopods suffered far greater extinction did spiriferid brachiopods… In summery, non of these patterns is consistent with the salinity gypothesis.”Species Area effectsIf you look back through the geological colum, you will find a corrolation between marine regressions and major mass extinctions. But what really is the connection. Erwin gives use a good base from which we can conclude many new things. His statements are based on MacArthur and Wilson’s theory of island biogeography.They suggested that species diversity on an island is a function of immigration to the island from a continental source, and extinction on the island due largely to competition. Thus the immigration rate should be a declining function of the number of species on the island and should approach zero when all the species from the source pool have reached the island. Similarity, as species diversity increases, the extinction rate should climb as competition for resources increases. The equilibrium species diversity will be the point where the immegration rate and extinction rate are the same. Among the implication of the theory are that smaller islands and more distant islands should have fewer species than larger islands or those closer to the source area” (Erwin, 1993).Using the species-area hypothesis we can deduce several facts. We already know that during the PTB the sea level declined and that one single continent was formed. This reduces shelf area and thus reduces the area a species can live in causing greater competition for resources. You will then get species dying off and lower species diversity. Some scientists claim that a reduction of shelf area alone would have cause the extinction. Since most organisms on land are connected to the sea, we can postulate that there would also be a reduction in the number of species on land.Many scientists, as reported by Erwin, have rejected the species-area hypothesis. Their rejection is based on many facts. Some point to an example during the Middle Eocene where there was a 50% reduction is shelf area along the Gulf Coast. According to the species area hypothesis there should have been a reduction in diversity of species but evidence supports that there wasn’t. Some argue that it is only the change in number of marine provinces that affects diversity. To me it would seem that there if there is a reduction in species area there should be a reduction in marine provinces are at least the area of space to live in each province. Erwin (1993) makes some very challenging suggestions.If the species-area relationship is valid, regressions should have a far greater effect on continents than on islands since, in general, the area of an island will increase during a regression. Modern tropical reef biotas are among the richest environments in the world, rivaling if not surpassing the tremendous diversity found in tropical rain forests. If most marine families have representatives on oceanic islands they will be relatively immune to regression-induced extinction.Anoxia-Stagnant OceanVolcanismThere is very good evidence that volcanism caused or triggered the PTB extinction. A paper by Paul R. Renne et al (1995) puts together numerous papers into one sound thesis. The evidence of a bolide impact and for volcanism were synchronous within sever hundred years. Thus more scientists believe in the volcanism theory rather than an asteroid simply because there is more evidence for volcanism and also that volcanism can produce some of the effects of an asteroid collision with earth which can eliminate the asteroid hypothesis all together. Erwin (1993) stated four ways in which volcanism might be able to cause mass extinctions. Creation of a dust cloud that reduces photosynthesis and initiates global cooling; injection of massive amounts of carbon dioxide and sulfates into the atmosphere into the atmosphere causing global warming.Creating acid rain as the sulfate is converted to sulfuric acid and reducing the protective ozone shield.Creation of a thermal anomalyInjection of poisonous trace elements into the atmosphere and oceans.The Siberian traps represent the most voluminous known continental flood voluminous known continental flood volcanism in Earth’s history, with an original volume estimated at 2,000,000 to 3,000,000 cubic kilometers distributed over 2,500,000 kilometers square in central Siberia. The traps’ volcanic succession overlies Permian strata and is in turn overlaid by Triassic strata….. (Rene et al. (1995) When Ar-40/Ar-39 and U-Pb dating is done on the volcanic rock, the age turns out to be 250 Ma with a plus or minus 1.6 Ma space. Many other dating techniques have been used and they all roughly agree on this date. Many scientific papers such as Wingnall et al. (1993), Dao-Yi et al (1993, 1989, 1995), and Erwin (1993) and the papers they used give irrifutable evidence of volcanism. Therefore there is scientific proof that a great volcanic event occurred during the PTB. But how could the volcanoes in Siberia have produced such an unpresidented global extinctiuon. Wignall (1993) give a short hypothesis on this subject….we suggest that the effect of huge volumes of carbon dioxide released during the eruption of the Siberian flood basalts may have led to global warming, which in its turn produced extensive areas of warm saline bottom waters poor in oxygen. The major negative swing of carbon isotopes in the early Griesbachian could be recording this major volcanogenic input of isotopically light carbon.Renne (1995) gives his interpretation.Siberian flood volcanism, perhaps augmented by sulfates derived from evaporites of the Siberian platform, could have produced sufficient stratospheric sulfate aerosols for rapid global cooling to ensue. Resulting ice cap accumulation likely caused the dramatic marine regression, which in turn led to subaerial exposure of the continental shelves. This latter effect would account for the ubiquitous anomalies in C, S, and Sr isotopes. Isotopically light C and S from mantle-derived carbon dioxide and sulfur dioxide would also contribute to the observed negative anomalies in C-13 and S-34. Ice storage effects plus enhanced erosion of the continental crust could have produced the seawater O-18 enrichments observed at the boundary. Rapid transgression after the boundary would follow from the abrupt cessation of Siberian volcanism and the resulting ice cap recession. Climate recovery may have been enhanced by slower devoloping greenhouse effects of volcanogenic gasses, primarily carbon dioxide. Indeed, a short-lived volcanic winter, followed within several hundred thousand years by greenhouse conditions, would fully explain the environmental extrema that caused the P-T mass extinctions. Pyroclastic EruptionsFlood BasaltsTrace element poisoningOtherThere are many other proposals that can explain the Permian extinction. I will describe some of the most intreging ones.The aftermath of the extinctionWe risk supersaturating our readers with the anomalies of evolution, but we simply cannot bypass an article that is introduced as follows: “An analysis of the fossil record reveals some unexpected patterns in the origin of major evolutionary innovations, patterns that presumably reflect the operation of different mechanisms.”The most interesting “unexpected pattern” is the gross asymmetry between the diversification of life in the Cambrian explosion (about 440 million years ago) and that following the great end Permian extinction (a little over 200 million years ago). Biological innovation was intense in both instances; both biological explosions burst upon a life-impoverished planet. Many niches were unoccupied. Even so, all existing (and many extinct) phyla arose during the Cambrian explosion and none followed the Permian extinction. “…why has this burst of evolutionary invention never again been equaled? Why, in subsequent periods of great evolutionary activity when countless species, genera, and families arose, have there been no new animal body plans produced, no new phyla?”Some evolutionists blame the asymmetry on the different “adaptive space” available in the two periods. “Adaptive space” was almost empty at the beginning of the Cambrian because multicellular organisms had only begun to evolve; whereas after the Permian extinction the surviving species still represented a diverse group with many adaptations. (Just how the amount of “adaptive space” available was communicated to the “mechanism” doing the innovation is not addressed.) Scientists contemplating these matters, however, seem to concur that microevolution, which supposedly gives rise to new species, cannot manage the bigger task of macroevolution, in particular the creation of new phyla at the beginning of the Cambrian. (Lewin, Roger; “A Lopsided Look at Evolution,” Science, 241:201, 1988.) 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A conodont-based high resolution biostratigraphy for the Permo-Triassic boundary interval. In: W.C. Sweet et al. (Editors), Permo-Triassic events in the Eastern Tethys – an overview. Cambridge Univ. Press, pp. 120-238.Tappan, H. 1968. Primary production, isotopes, extinctions and the atmosphere. Palaeogeography, Palaeoclimatology, Palaeocology 4:187-210.Wingnall P.B., and Hallam, A. 1993. Griesbachian (Earliest Triassic) palaeoenvironmental changes in the Salt Range, Pakistan and southeast China and their bearing on the Permo-Triassic mass extinction. Palaeogeography, Palaeoclimatology, Palaeoecology, 102: 215-237.Xu Dao-Yi, and Yan Zheng., 1993, Carbon isotope and iridium event markers near the Permian/Triassic boundary in the Meishan section, Zhejiang Provence, China. Palaeoecology, 104: 171-176.Xu, D.-Y., Ma, S,-L., Chai, Z.-F., Mao, X.-Y., Zhang, Q.-W. and Yang, Z.-Z., 1985, Abundance variation in iridium and trace elements at the Permian/Triassic boundary at Shangsi in China, Nature, 314 (6007): 154-156.Xu, D.Y., Zhang, Q.W., Yan, Z., Sun, Y.Y., Chai, Z.F., and He, J.W., 1989. Astrogeological Events in China. Geological Publishing House, Beijing, Von Nostrand Reinhold, New York and Scottish Academic Press, Edinburg, 264pp.These sources mostly have come from the University of Alberta. I used the “Gate” and databases to get periodicals and books. Some of the sources came from the internet. I emailed some of the authors and they sent their papers to me by email. Some I got from web sites. I have no web sites posted in my sources because all the only information I took was when the paper and its source was displayed. I didn’t quote or use internet sources because they are not reliable and most are based on opinions and not science.

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The End Permian Mass Extinction. (2018, Dec 23). Retrieved from https://graduateway.com/the-end-permian-mass-extinction/

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